1. Field of the Invention
The present invention relates to a device for controlling air-fuel (A/F) ratio of an internal combustion engine, more particularly, to an A/F ratio controller which corrects voltage to be supplied to the heater for heating the A/F ratio sensor according to the engine-driving condition.
2. Description of the Prior Art
When operating an internal combustion engine, in particular, one which drives a vehicle engine provided with a ternary catalyzer for purifying exhaust gas, the A/F ratio of exhaust gas must be strictly held at the theoretical A/F ratio. Today, there is such a specific A/F ratio controller available for use, which executes feedback control of A/F ratio by means of an A/F ratio sensor which sharply varies the level of output by applying the theoretical A/F ratio in order that the actual A/F ratio can approximate the theorectical A/F ratio.
Nevertheless, since the A/F ratio sensor of the abovecited A/F ratio controller can only measure the theoretical A/F ratio, actually, this controller cannot execute feedback control of A/F ratio covering an extensive range. To compensate for such disadvantage, recently, a preceding art presents a system for controlling the A/F ratio using an A/F ratio sensor which is capable of measuring not only the theoretical A/F ratio, but can also continuously measure the A/F ratio from the rich to the lean degree according to the volume of specific component like oxygen present in the exhaust gas. This A/F ratio sensor incorporates an oxygenconcentration detecting element composed of ion-conductive solid electrolyte and a heater which activates the element. Unless held at the predetermined temperature by means of a heater, the oxygen-concentration detecting element of the A/F ratio sensor is it cannot function correctly by itself. FIG. 1 is the graphical chart designating the relationship between the temperature of the oxygen-concentration detecting element and the deviation of signals outputted from the above-cited A/F ratio sensor (.DELTA.A/F). As is clear from this chart, independent of differential values of temperature borne by the oxygen-concentration detecting element against the predetermined reference level, deviation is generated by signals outputted from the A/F ratio sensor.
On the other hand, depending on the engine driving condition, the temperature of exhaust gas varies, and thus, the temperature of the A/F ratio sensor set to the exhaust-gas tube also varies. To compensate for this conventionally, the caloric value of the heater is controlled according to the load and the number of the rotational of the engine. Nevertheless, although the temperature of exhaust gas instantly responds to the engine driving condition, the temperature of the A/F ratio sensor does not instantly respond to the exhaust gas temperature. Conventional A/F ratio sensors cannot maintain the temperature of the oxygen-concentration detecting device at the predetermined value since they merely apply the variation of the load and the number of the rotation of the engine to the control of the heater. Consequently, error is easily generated in the signal outputted from the A/F ratio sensor, and as a result, the A/F ratio controller cannot precisely control the A/F ratio.